1. Field of the Invention
An embodiment of the invention relates to a method of accurately determining the spatial location of an RFID tag in two-dimensions or three-dimension. More particularly, the method utilizes a plurality of RFID readers to make a plurality of distance, direction, and or time-of-flight determinations. Such determinations are made by sending a request signal from one of the plurality of RFID readers and listening for a response signal from an RFID tag received at each of the plurality of RFID readers. Correction factors are then determined and the time-of-flight factors adjusted. The adjusted time-of-flight factors are then used to determine more accurately the distances between the RFID tag and each of the plurality of RFID readers. These more accurate distance measurements are then used to determine the spatial location of the RFID tag.
2. Description of Background
Before our invention physically locating an RFID tag in a warehouse was difficult and very imprecise. Radio tags (also referred to as RFID tags) are a technology that, when queried at radio frequency with a request signal reply at radio frequency with a response signal. RFID tags may be ‘active’ with their own power source or ‘passive’, drawing power from the request signals. The request signals and response signals may be of a fixed preset code or may contain dynamically generated information.
One problem with current RFID tag location schemes is that in a warehouse full of RFID tagged items RFID readers may capture RFID tagged inventory items that are on hand nearby but cannot precisely pinpoint the location of the items. In most cases RFID readers only capture the encoded data on the RFID tag response signal and maybe the RFID tag signal strength, which can only be used for a very rough approximation of the distance (not direction) between the RFID reader and the responding RFID tag.
Not being able to quickly locate inventory in a warehouse results in delays in inventory handling and as such problems in supply-chain management. As an example, when items are hard to find, warehouse shipments may be delayed. Supply chain delays may cause unintended consequences. First, the supply-chain can be delayed with immediate consequences of manpower and delays to find missing inventory items. Next there can be consequences resulting from low inventory levels downstream in the stores. Then finally upstream supplier consequences can occur resultant from the delay and inventory level problems causing suppliers to ship too many or too few inventory items to meet phantom inventory requirements.
RFID response signal strength can be used as a rough estimate in determining the distance between an RFID tag and an RFID reader, but cannot identify the specific direction from the RFID reader. In addition, the signal strength is not always accurate. Problem related to signal strength measurements can be that the RFID response signal may be reflected and that intervening objects (radio frequency (RF) obstacles) may block, degrade, interfere with, and or modify the RFID response signal strength. As such, RFID response signal strength alone is not a good enough measurement to reliably and precisely determine the location of a specific RFID tag.
Another method of determining the distance between an RFID tag and an RFID reader is by determining the time-of-flight starting when an RFID reader sends a request signal and ending when the RFID reader receives a response signal from an RFID tag. In this regard, an RFID reader can use the time-of-flight determination to calculate the distance between the RFID reader and the RFID tag. The problem here is that there is latency between the request signal and response signal. This latency occurs while the RFID tag is processing the received request signal. Factors such as manufacture specifications, temperature, and RFID tag characteristics all contribute to a turn-around delay in the RFID tag response signal.
This turn-around delay can have a profound effect on the ability to determine the distance between the RFID reader and the RFID tag. For example, if the turn-around delay in the RFID tag response signal takes an extra microsecond to reply beyond what the RFID reader is estimating, then the RFID reader will estimate the RFID tag to be about 150 meters farther away than it really is.